1. Field of the Invention
The present invention relates generally to the art of plastic bag making machines, and more particularly to bag machines which employ a rotary sealing drum. Still more particularly, the invention relates to a plastic bag making machine which produces a novel type of folded bag which is partially cut and partially perforated to provide for the easy dispensing of bags.
2. Description of the Prior Art
Many different types of plastic bag making machines are known to the art for producing plastic bags for industrial and individual consumers for many different applications (e.g. small sandwich bags and trash bags). While the present invention has a wide range of applications for the production of such products, the prior art will be explained by reference to one particular class of bags, i.e., polyethylene trash bags or, garbage bags and wastebasket liners of the type usually sold in boxes of folded bags or rolls of bags.
In the manufacture of such products, it is first necessary to prepare a plastic film tube, which is usually accomplished by an extruder. Heat and pressure are applied to pellets of polyethylene to melt the starting material which is then forced through a ring die. As the polyethylene leaves the die, air is injected through the center of the die to form a tube or bubble. This process is known as the "blown tubing" process.
The tube is then cooled by air or water rings so that the hot plastic sets. The tube is then conveyed to nip rollers which flatten the tube for subsequent processing. The tube at this point is seamless and is ready for downstream processing. The particular plastic starting material, the diameter of the tube, the thickness of the plastic and a wide variety of other variables may be controlled by the manufacturer for a particular end use application.
By downstream processing is meant the various manufacturing steps which take place after the tube is formed. They may take place as the plastic tube is extruded, in which case the process is called an "in-line" process. On the other hand, they can take place "out of line" using rolls of tubing produced by an extruder at another location in the plant or using rolls produced at another locality. The two main downstream operations are the formation of the bags themselves and the packaging of the bags thus produced. It is the former operation with which the present invention is primarily concerned. The prior art relating to this operation will be described in greater detail below. With regard to the packaging operation, this is usually accomplished by folding individual bags so they fit into a box or by rolling the bags into a roll (either over a core or in the form of a coreless roll). Rolls are typically placed in a bag or folding carton for sale.
The rolling of individual bags into a roll has certain advantages to the consumers because the bags dispense one-at-a-time. The preparation of such rolls of individual bags, however, is difficult. A prior art patent which describes such a roll of individual bags is U.S. Pat. No. 3,826,361 issued to William F. Heckrodt on July 30, 1974 for "Plastic Bag Dispenser System."
Other conventional types of rolled bags are easier to prepare than the rolls of individual bags. These rolls have a perforation over the width of the bags adjacent the transverse bag forming seal which must be torn apart by the consumer. The separation of the bags may be difficult under certain circumstances because the location of the perforation may be hard to find, for example, under poor light conditions. A bag system from which individual bags could be easily separated from a continuous roll would be a significant advance in this art.
Proceeding now to a more detailed description of the bag making operation itself, the various known prior art devices are designed for either in-line or out-of-line operation and involve two separate operations on the tube. The first is to form a seal across the width of the tube by a sealing bar. The bar, under the influence of pressure, melts the two faces of the flattened tube and seals them together. The second operation is to perforate the sealed tube, which is usually accomplished by a bed and fly knife perforation assembly. The perforation is also provided across the width of the bag and usually adjacent to but slightly spaced apart from the seal.
Many problems are encountered in optimizing the design of a bag making machine. First, the machine must be capable of running at high speeds, a problem which is becoming more pronounced as extruder technology advances. Modern extruders are capable of producing film at a rate of 400 feet/minute or more, yet commercial bag machines are not capable of keeping up with film tube production.
Second, because the formation of the bag seal involves melting of the plastic, the bag machine must be able to handle the film at high speed without pulling the heated seal apart.
Third, it is difficult to maintain the correct distance between the seal and perforation (the "skirt length") as the film proceeds through the machine.
Fourth, if it is desired to produce different sizes of bags on a single piece of machinery, the techniques involved in prior machines for changing bag size are cumbersome, expensive and time consuming.
Fifth, prior art bag machines do not make bags which can be easily separated from a roll.
A more detailed description of several prior art bag machines will illustrate these problems. Most commercial machines today are of the shuttle or oscillating type, where the film stops for a brief period of time while the perforation and sealing operations are performed. While the film is stationary, heat and pressure are applied and a small amount of dwell time is provided to allow the seal to cool. While such machines have proven to be reliable and commercially acceptable, film extruder speeds are now much higher than this type of bag making equipment can handle. Moreover, since these machines usually require a seal width of at least 33 inches, the oscillation mechanism becomes bulky, and only a limited amount of running time can be accomplished without mechanical breakdown of the oscillation mechanism. However, the main disadvantage of this type of machine is running speeds, with 280 feet/min. being the fastest oscillating line speed known to the present inventor.
Another type of known bag making equipment operates on a rotary principal in which a sealing drum rotates on a shaft and the film is maintained in contact with the drum by use of a tensioning web or sealing blanket. The seal is made by one or more sealing bars located at the surface of the rotating drum. The surface of the drum is usually coated with rubber lagging.
In rotary bag machines, such as the Polyrocket machine, the drum size and number of sealing bars determines the bag size, and, accordingly, to vary the bag size involves drum replacement. Drum changes frequently take 2-4 hours and require skilled mechanics to reposition the sealing blanket, electrical systems, drives, etc.
Another problem with prior art rotary machines has to do with the seal, and arises from the fact that the "hot" seal is transported in the machine to the perforation section. This must be accomplished very carefully to avoid stretching and weakening of the seal. Because the film requires both perforation and folding after the seal is formed, any tension applied to the film downstream of the sealing mechanism would be transferred back to the seal area causing the aforementioned stretch and weakening problems.
Finally, in prior art rotary machines, the perforation section is followed by folder boards which reduce the width of the bag from about 34 inches to a packaging size of about 81/2 inches (by folding the flat tube twice) for sale in conventional retail size boxes or rolls. To locate the folding boards after the perforation section requires additional space in the manufacturing area.
A bag making machine which overcomes the aforementioned disadvantages of prior art machines would represent a significant advance in the art.